Conventionally, Japanese Laid-open Patent No. 4-330314 describes a catalyst type exhaust emission control device for a diesel engine, which reduces and decomposes NO.sub.X contained in the exhaust emissions from a diesel engine. According to Japanese Laid-open Patent No. 4-330314, the device is composed of a copper-zeolite NO.sub.X catalyst provided in an exhaust path of a diesel engine, a hydrocarbon sensor for detecting the hydrocarbon concentration in the exhaust emissions, and a fuel spraying means which is provided in the exhaust path at the upstream side of the hydrocarbon sensor and which sprays fuel into the exhaust path to control the hydrocarbon concentration in the exhaust emissions within a predetermined range. According to the aforesaid configuration, based on a detected value of the hydrocarbon concentration, fuel is appropriately sprayed into the exhaust path from the fuel spraying means, and the hydrocarbon concentration in the exhaust emissions is kept within the predetermined range. The fuel sprayed from the fuel spraying means acts as a hydrocarbon reductant, and activates the copper-zeolite NO.sub.X catalyst in association with oxygen contained in the exhaust emissions from the diesel engine. Thus, it is proposed that the reduction of NO.sub.X in the exhaust emissions is promoted. However, the copper-zeolite NO.sub.X catalyst has a low performance in controlling NO.sub.X.
Accordingly, in order to obtain a higher performance in controlling NO.sub.X, a large quantity of hydrocarbon for reduction is necessary, and this is disadvantageous in the operational cost of the catalyst.
On the other hand, an NO.sub.X catalyst containing an alkali-earth or a rare-earth oxide and a precious metal, and an NO.sub.X catalyst having silver as an active component show a higher efficiency of controlling NO.sub.X with a smaller quantity of reductant hydrocarbon as compared with a copper-zeolite catalyst.
However, the high-performance NO.sub.X catalyst containing an alkali-earth or a rare-earth oxide and a precious metal, and the NO.sub.X, catalyst having silver as an active component enable highly efficient control of NO.sub.X, but have a disadvantage in that they have a lower durability against SO.sub.x, and therefore it is difficult to put them into practical use.